The invention relates to a belt tensioner for a generator belt drive of an internal combustion engine. The belt tensioner comprises:                An annular housing part that is mounted on the generator housing so as to enclose the generator shaft,        An annular tensioning part that is supported on the housing part so that it can pivot about the generator shaft,        A single tension pulley that is supported on the tensioning part and pretensions the belt,        And a spring that is clamped between the housing part and the tensioning part and applies a torque to the tensioning part for generating the belt pretension.        
Such a belt tensioner is known from EP 2 573 423 A1. The spring is constructed there as a helical spring surrounding the generator shaft.